In a semiconductor transistor, regions of source/drain (S/D) in the vicinity of a gate are formed of Si (silicon), SiGe (silicon germanium) or the like. In a manufacturing process of the transistor, there is a need to remove a silicon oxide (SiO2) film as a native oxide film that is formed on surfaces of the S/D regions, after forming contact holes. A variety of insulating films such as an SiN (silicon nitride) film for controlling a dielectric constant are formed in the vicinity of the gate. As such, etching of the native oxide film with a high selectivity to the SiN film is required. Here, a chemical formula of the silicon nitride is abbreviated as “SiN” without considering the stoichiometry.
A method based on a chemical oxide removal (COR) process using an HF (hydrogen fluoride) gas and an NH3 (ammonia) gas has been known as a method of removing an SiO2 film as the native oxide film. The etch of the SiO2 film formed on a surface of a semiconductor wafer requires supplying a mixed gas of the HF gas and the NH3 gas into a processing vessel and producing an (NH4)2SiF6 (ammonium fluorosilicate) gas through the reaction of the mixed gas with the silicon oxide. The SiO2 film is removed (or etched) by heating and subliming the (NH4)2SiF6 gas. However, this method causes a problem in that the SiN film also reacts with the HF gas and the NH3 gas, thus being etched.
In recent years, there is a method of forming an SiN film at a relatively low temperature so as to suppress damage to a surface of a semiconductor substrate. In such a low temperature, the SiN film tends to be a porous film. This results in the increased tendency to etch the SiN film, thus causing a need for a significantly high level of selectivity.